Electrical fault finder



Sept. 8, 1953 A. J. DEVOT ELECTRICAL FAULT FINDER Filed Jan. 7, 1948 3 Sheets-Sheet 1 BLOCKING GATE SWEEP OSCILLATOR CIRCUIT CIRCUIT 4% W 4 9 7 i 7 l] 1 lNv T Ra RANGE PULSING CIRCUIT {AL- CATHODE MARK I FOLLOWER (f) R IT L gem CIRCUIT 3 FOR ALL -'|2 L "'10 F UNITS ll OSCILLOSCOPE TUBE 5"CATHODE RAY TUBE FIG. I

84 84 5 83 IIIIIIl ||I lll|2 HIIIIH'HHIHH'IHI G. 4

F I j 7 TM god-T FAULT LOW, VOLTAGE HIGH VOLTAGE FINDING H|GH CURRENT D-C. POWER PPARATUS .D SUPPLY SUPPLY A lNl/E/VTOR POWER FIG 5 Andre fp'af SUPPLY 87 By fi/MW HIS ATTORNEY p 8,1953 A. J. DEVOT 2,651,752

ELECTRICAL FAULT FINDER Filed Jan. '7, 1948 s sheets-sheet g FIG.2I

HIS ATTORNEY Sept. 8, 1953 1 DEVOT 2,651,752

ELECTRICAL FAULT FINDER Filed Jan. 7, 1948 3 Sheets-Sheet 5 1s ATTORNEY Patented Sept. 8, 1953 UNETE ELECTRICAL FAULT FINDER Andr J. Dvot, Canton, Mass., assignor to Tobe Deutschmann Corporation, Canton, Mass., a

corporation of Delaware Application January 7, 1948, Serial No. 997

13 Claims.

The present invention relates to an electrical fault finder for locating the position of faults on electrical lines and cables. Electrical faults may be in the nature of circuit grounds, a break in the circuit, or change of circuit characteristics of such a nature as to affect the efliciency of power transmission, of signal or speech transmission, or the operation of the circuit for other uses.

Various systems and devices have been develcped for the location of electric faults on lines, including resistance and inductive measurements. In the present invention, results are ac complished through means employing a short, in fact an ultra short, electric pulse which is transmitted down the line from a given point of observation and in which the time of travel necessary for the pulse to reach the fault and be refiected back in the vicinity of the point of origin is measured to indicate the length on the line to where the fault occurs.

In the present system, a measuring standard is set up by timing circuits, the operation or" which is initiated with the transmission of a short pulse sent down the line. This measuring circuit includes means for providing suitable measuring markers on a cathode ray oscilloscope tube on which the faults are visibly indicated by means of peaks formed by the beam in the tube and read in relation to the marking points.

The present invention may be used to locate faults on many kinds of lines and cables of greatly varying lengths and to locate the position of the fault accurately. It is possible also to determine whether the fault is of an open or short circuit nature or merely a change in the constants of the circuit.

Further advantages. and improvements provided by the present invention will be more clearly understood from the description of an embodiment of the invention in the specification annexed hereto, in which Fig. 1 shows a block diagram of the circuit employed;

Fig. 2 shows a schematic diagram of the circuit shown in block in Fig. 1;

Fig. 3 shows a series of curves indicating the function of the various units in the circuit.

Fig. 4 shows the face of the cathode ray oscilloscope with indication shown; and

Fig. 5 shows a detail for maintaining certain desirable conditions in a line under test.

In Figs. 1 and 2, I indicates a line under test, which may be a multiconductor cable, a concentric cable, single power or signalling line,

multiple power or signalling lines, or, in fact, any type of line circuits or continuing network in which a considerable extent of line is employed and is to be tested for electrical fault. The range of the apparatus may be made to extend as high as 40 or miles or even higher.

Referring initially to Fig. 1 in connection with the curves of Fig. 3, the system may be controlled or set in operation by a blocking oscillator 2, which may have a repetition rate of somewhere in the neighborhood of 400 to 600 cycles per second. The function of this blocking oscillator is to produce a sharp peak as indicated by the curve a (Fig. 3), in which the peaks are of the order in width of one microsecond, and spaced about of a second apart where the repetition rate is 400 cycles per second. The blocking oscillator which, as noted,controls the repetition rate of the system is connected to a so-called gate circuit 3, whose function is to produce a square topped wave as indicated by the curve I), the forward side i of which corresponds in time to the vertical line of the blocking peaks of the curve a. The gate circuit provides a negative square waved pulse, the width of which covers the time interval of one entire measuring operation. The gate width 5 of wave b (Fig. 3) is made sufiiciently wide, therefore, for whatever range of measurements for which the device is to be used. If the device is to cover a range of 40 miles, the gate width 5 will then, of course, be double the range for 20 miles, for example. The gate circuit 3 operates or controls three unit circuits, the sweep circuit 6, the range mark circuit I, and a so-called gate inverter and cathode follower circuit 8, the function of the latter being to invert the gate pulse from a negative to a positive wave or si nal as indicated by the curves f (Figs. 1 and 3). The positive gate ouput is fed to the pulsing circult 9 and to the intensifier grid ill of the oscilloscope tube II.

A receiving circuit I2 is connected with the line to receive the direct and reflected pulses, while the output of the receiver is connected to the control element of the oscilloscope tube I I to indicate the reflected signal in its proper time related position.

The ouput of the gate circuit which is im pressed upon the sweep circuitfi functions to start the operation of the sweep circuit at an instant corresponding to the forward line 4 of the gate wave in the curve I), and to stop the operation of the sweep circuit at the other side, 4, of the gate wave curve. As a result, the output of the sweep circuit indicated by the curves 0 and (I each has a width the same as the width of gate curve b for both the positive and negative sweep pulses c and (2, respectively. The positive and negative of the sweep pulse outputs permit a convenient means of centering of the cathode beam trav l across the tube. This is obtained by construct ing the sweep circuit with a push pull output, which produces the positive and negative sweep waves.

The negative gate output 13, when impressed upon the range mark circuit 1, produces an output curve 6 comprising a series of range mark peaks which mark off or divide the time width 5 of the gate circuit output uniformly. The d1- visions, for instance, may correspond of a mile or a half mile. Ten such marks, at a distance corresponding to a-mile apart, would give a measurement rate of 10 miles, which on open Wire equals a time interval of 109:2 microseconds. means that the gate width 5, under such conditions, will correspond to such a time interval. Thecurve of the output oi the gate inverter is the-same as the curve-b with the exception that the curves are positive instead of negative. When the positive gate wave is impressed upon the pulsing circuit, sharp peaked pulses are produced in the output of the pulsing circuit, each pulse beginning with the forward end of the positive gate and closing very sharply to make a peaked pulse long before the end of the gate pulse, preferably a fraction of a microsecond after beginning. Such a peaked pulse may, if desired, be produced at substantially high voltage so as to ionize or arc the fault in the circuit'in order'to maintain the low resistance characteristic fault of the circuit. This will be discussed more fully in other parts of the specification, and the same effectmay be obtained by means described in connection with Fig. 5.

Referring more particularly to the circuit diagram of Fig. 2, the section :2 enclosed by dotted lines shows the blocking-oscillator corresponding to the block diagram of Fig. 1.

As indicated by the diagram, this may be-operated with a triode tube '26. Inthis circuit, an initial positive potential impressed upon the .grid 2i rapidly produces a negative potential through the charging of thecondenser 22 to cut off the tube conduction and produce a sharp peaked curve. The frequency of repetition is regulated the time it takes 'ior the condenser 22 to discharge to permit the tube to become conductive again. The output of the blocking oscillator curve a (Fig. 1), provided through the secondary 2 3 of the transtormer'm, is impressed through the condenser on the gate circuit 2b, which may also be a double 'triode tube or two individual tubes, if desired.

In the gate circuit v2, current normally flows between the cathode 2i and the anode 28. W hen, however, the positive pulse from theb1ock ing oscillator is impressed upon-the grid .29, the grid takes over, acting to cut off the current between the cathode 2'! and the anode '28,thereby permitting the other half of the tube to become conductive until the charge has leaked ofi the grid 2%) through the discharge of the condenser 25. The pulse obtained from the gating tube circuit in the form shown in curve I) is impressed through the output'line 38 on the sweep circuit 5, the gate inverter 8, and the range marking circuit l. The sweep circuit 6 receives the gate pulse through a condenser '3! which is connected to the grid 32 of the input tube 33 of the sweep to distances laszthezwidthiof :the negative gate.

4 circuit. The forward end of the pulse from the gate circuit cuts off the current flow in the tube 33 and permits the condenser 34 to charge through the tube 35, the cathode of which, in the function of a cathode follower, controls the sweep circuit. An additional condenser 36 and resistance 3.! ,connected .together in shunt and series with thercondenser -34 sharpen or quicken the building up of the charging or voltage in the initial part of the sweep circuit in order to make this portion of the sweep curve more linear.

"ThEI'BEIJIEQHd of the gate pulse acts to make the tube 33 again conductive, which sharply cuts off :the charging of'the condenser 34 to end the sweep .pulseonbring the voltage back to zero. The sweep pulse, therefore, has the same length Between the cathodefollower tube 35 and the input tube 33 is an amplifier tube 38 which may operate as a class A type and acts'to increase the potential efiect -built-u-p on the cathode follower tube. The

cathode 39 of "the cathode follower tube 35, it will be noted, is directly connected to the plate dfi-of the sweep 'cirouit. The rest of the sweep circuit includes-a potentiometer device 4|, which acts as an expander for expanding the travel of the cathode beam by =regulatingthe magnitude of thesweep potential as applied to the control plate of the oscilloscope. A centering circuit is also employed Lin connection with a push-pull amplifier stage v involvingvacuum tubes 2 and 43. Thiscentering circuit shown at :34- comprises apotentiometerfiAS with an adjustable tap 15 which controls itheisweep circuit outputs of the tubes #2 'and.-i43,.one.of which is positive andithe other negative asidelivered by the lines t! and 23 'to the two plates 49 and 5B of the oscilloscope tube for-sweeping the beam in one linear direction across :the face 5! of the tube. The sweep voltage delivered to the oscilloscope tube causes the beam to sweep across the tube in, for instance ae herizontal.direction at a rapid rate correspondingato the variation'in potential produced by the .sweep -:-and to .be repeated in ac cordance with the blocking oscillator control. The rateiof .the:sweep, however, must be adjusted to conform with .theztr-avel of the electric pulse over-the electric lineonnetwork to be tested. As is .welliknown,theivelocityoftravel of the pulse in the network is of the order of the velocity of light, approximate1y:18,6-,000 .miles per second or less, beingsomewhatlessior the transmission of pulsesoverlines :orpables'than in free space because of the retardation effects of the line. The sweep of the :beam :across the .face of the cathode :ray ;osci1losoope,iif tmade to correspond to a time measurement having a tenimile line, will be of theprder of 19,-9.2.microseconds for a typical open wirefline.

This time intervalitself is-geXt-remely short, but by means of -the ipresentiinvention range marking pips are impressed upon -th e beam to indicate intermediate :distances on the face or" the cathode ray tube. These range marking pips are obtained by pulse voltages impressed upon the other plates 52 and .53 .of the cathode ray tube. lithe-plates 49rand't50 produce a horizontal travel of thebeam, the :plates52 and 53 will cause a vertical-travel and'corresponding mark of the beam. .Thezrange marking circuit is en closed within :the .area 1. This circuit receives the negative gate :Wave over the line 54, which connects through the-condenser 55 to the grid 56 of the double triode 5-1. The grid 56 of the tube 51-cutsofi at the Iorward end of =thegate pulse,

permitting the other half of the triode or independent tube (if one is used) to oscillate with the circuit 58 acting to control the rate of oscillations through the remaining operating portion of the gate interval. Vernier controls are obtained through the circuit 59 by which the oscillating period may be accurately adjusted. The first of the oscillations produced in the output of the tube 51 is therefore synchronized with the initial operation of the gate pulse. The frequency is continuously variable in the nature of 40 to 70 kilocycles for one mile markers on cables and two mile markers for open wire lines. These oscillations may be passed through an amplifier stage including the tube 60 from which they are impressed on a blocking oscillator circuit including the tube 6 l, which is similar to the blocking oscillator previously described, inasmuch as each oscillator is peaked to a point in the output of the tube SI, which is connected with the plates 52 and 53, the latter through ground of the cathode ray tube.

The forward end of the gate wave commences the operation of the range marking circuit, thereby producing the range marking pulses, while the rear side of the gate wave sharply cuts these marking pulses off, so that for each gate wave a certain number of range marking pulses are produced. The frequency of oscillation in the range marking circuit may be such that the marking pulses are set up for a half mile or for every inile, if desired, or for some different magnitude. The range marking pulses act to cause the beam to travel transversely to its normal travel in its sweep across the tube. Since the sweep circuit is expanded or contracted through control of the sweep itself, the range marking pips will also be expanded or contracted by the same means. Their position is, therefore, always relatively correct with that of the sweep circuit.

In addition to the elements which have been described, two other circuits are synchronized with the gate circuit but through the gate invertor, which converts the negative gate pulse output to a positive pulse as already mentioned and shown in curve 1. The gate invertor which serves this purpose receives the negative gate input over the line 54 and condenser 62, through which the negative pulse is impressed upon the grid 63 of the tube 64. The tube 65 in the gate circuit acts as a cathode follower with the tube to produce a positive pulse corresponding to the negative pulse over the output line 66. This positive pulse is impressed on the pulsing circuit and on the intensifier or suppressor grid ll) of the oscilloscope tube II.

The positive pulse output 66 from the gate invertor is also fed to the pulsing circuit which time interval corresponding to the travel of a pulse to the end of the fixed line and return; By that time the line has discharged and the tube is extinguished. The pulse line is charged up through a high resistance 12' which with the line forms a comparatively slow time circuit,

the resistance preventing rapid recharging. The length of the line 12 controls the duration of the pulse. This may be short, of the order of .5 of a microsecond, or considerably longer where desired.

The pulse transmitted over the cable or network 13 to be tested is picked up, after reflection from the point of failure therein back to the head of the line or the place where the pulse was first impressed, by the line 14 through which the refiected pulse together with the direct pulse is impressed on the first receiver tube 15.

The receiver circuit, which may be omitted only if the reflected pulse has suflicient energy to operate the indicator itself, steps up the reflection through an amplifying and power stage for impressing the input of tube 18 by means of the line upon the plate 52 of the oscilloscope tube. The other vertical plate 53 is connected through the vertical centering potentiometer or network 59 to the same line Tl through a substantial resistance 80.

It will be seen that both the range marking circuit and the receiving circuit impress their inputs across the same control plates or elements of the cathode ray tube. Each functions to produce pips or serrations in the cathode beam image on the screen transverse to the normal course of the beam controlled by the sweep circuit.

Since the pulse impressed upon the grid Iii of the cathode tube is positive, the beam is intensified during the gate circuit interval and suppressed during the rest of the interval in which conditions are established for the next operation. Since the screen of the cathode ray tube has a persistence and since the whole operation is repeated 400 to 600 times per second, the positions of the pips and range marks are stationary and visible and may be easily read. This is indicated in Fig. 4, where 8| is the face of the cathode ray tube oscilloscope, 82 is a marking scale or rule calibrated in distance if desired, and 83 shows the trace of the cathode beam. The pips or marks 84, 84 etc. represent the range marks produced by the beam, while the mark 85 represents the reflected echo received from the receiver. It will be seen that the distance of the fault on the line may be read off directly in distance.

In the circuit of Fig. 5, a constant D. C. po-

tential is maintained on the line through a power source 85 of low voltage and high current, which may be separate from the power source 31 for energizing the rest of the system. The purpose of this separate power source 86 is to provide a potential such that an arc produced by the fault under ordinary operation will be maintained under test. It is preferable, however, to have an additional high voltage D. C. source which may have a low amperage capacity for discharging a high voltage across the gap 90 to produce an arc at the fault F which will be maintained by the low power source 86. Where an arc has occurred, it often happens that when the power is removed the fault will be a high resistance rather than a low one, making it difficult to find the fault, particularly where the break is from line to ground which normally is a high resistance. The line is therefore maintained under a potential during test sufficient to provide simulated operating conditions under which the fault ocours. The pulse then sent down the line will be more readily reflected.

Having now described my invention, I claim:

1. A system for locating a fault on an electrical line comprising means for producing a square acsnvsa topped electricpulse-of desired duration andfrequency, the forward side of the pulse marking the initiation of operation and the rearward side marking the termination of operation, a sweep circuit means, a range marking circuit means for producing a number of ,marking peaked waves spaced uniformly with the initial peak correspending to the initiation of operation, each: of said means being connected to said first named means, a cathode .ray oscilloscope tube having a cathode ray beam with means for controlling the movementthereofa receiving circuit connected to saidline for receiving the reflected pulses from the line, means connecting said sweep circuit to saidtube for moyingthebeamacross the tube, and means connecting .said range circuit and receiving circuiteach to said cathode ray tube for moving the beam transverse to the previous mentioned motion of the beam for producing range marksand location marks of faults.

2. In a system for locating a fault on an electricalline, a yisualcathode ray oscilloscope tube having, mean,s for producing a linear travel of the beam in diametrically opposite directions, a. gate circuit means, a range marker means operatively connectedto thencathode ray tube for producing a seriesof marks with the beam on said cathode raytube, a pulse producing means connected withsaid line, a sharp peaked oscillator.operativelyv connected to the gate circuit means, and connecting .means between the gate circuit, means and the pulse producing means, andthe range marker-means to start the pulse production andto startand stop respectively the series of ,marks, on. they cathode ray tube.

3. Ina systemfor locating a fault, on an elsetrical line, means connected to said line for producing a peaked power, pulsev of less than a microsecond duration, a cathode ray oscilloscope tube adapted to produce an electron beam made visible by a screen, sweep circuit means connected to saidtube for controlling the movement of the beam across the tube as a known function of time, a gate circuit for producing a square topped wav,e, mean s ,connecting the gate circuit to the power pulse meansfor-triggering the power pulse and the beginningv of the sweep. with the forward end ofthe square topped wave and ending the sweep with the rear end .of the square topped wave.

4. In a system for locating a fault on an electrical line, means for producing a peaked power pulse of less than: a microscecond duration, a cathode ray oscilloscope tube adapted to produce an electron beam made visible by a screen, sweep circuit means connected to ,said tube for controlling the movement of the beam across the tube as a known function of time, range marking means connected to said tube for controlling the beam to produce range marks during its sweep, a gate circuit for producing a square topped wave, means connectingthe gate circuit to the power pulse means-for triggering the power pulse, the beginning of the sweep, and the range marking meanswiththeforward end'of the square topped wave and ending the sweep and operation of the range marking meanswith the end of the square topped wave.-

5.' In a system for-locating ;a fault on an electrical line, means; for producing a peaked power pulse of less than a microsecond duration, a cathode ray oscilloscope-tube adapted to produce an electron beam madevisible by a screen, sweep circuit meanslo nnectedz-to said. tube f r c nt l:

8= ling the-movementgof the beam across the tube as a known-function of time, range marking means connected to said-tube for controlling the beam to produce-range marks during its sweep,

, :a gate circuit-for producing a square topped wave,

means connecting. the gate circuit to the power pulse means for triggering the power pulse, the beginning of the sweep, and the range marking means with the forward end fo the square topped wave and ending the sweep and operation of the range marking means with the end of the square topped wave, and means for expanding the travel of the sweep across the tube whereby the distance between range marks may be adjusted.

6. In a system, for locatinga fault on an electrical line, means for producing a peaked power pulse of less than a microsecond duration, a cathode ray oscilloscope tube adapted to produce :an electron beam made visible by a screen, sweep circuit means connected to said tube for controllingthe movement of the beam across the tube as a known function of time, range marking means connected to said tube for controlling the beam-to produce range marks during its sweep, a gate circuit for producing a square topped wave, means connecting the gate circuit to the power pulse means for triggering the power pulse, the beginning of-the sweep, and the range marking means with the forward end of the square tcpped'wave and ending the sweep and operationof the range marking means with the end of the square topped wave, a calibrated scale on the face of the cathode ray tube and means for expanding the travel of the beam to adjust the marking positions to corresponding calibrations of the scale.

'7. In a system for locating a fault on an electrical line, potential means for maintaining sufiicient direct current power on the line to sustain a fault arc thereon,- means for impressing periodic peaked pulses on the line, means connected to the line for receiving the peaked pulses after reflection from the line, a cathode ray tube having means for producing a cathode beam, means operative to control said-beam for indicating a time -measuremen-t, connecting means from said means operative to control said beam to said receiving circuit whereby said peaked pulses'and reflections will produce an indication of a time measurement by operation on said beam.

8. In a system for locating a fault on an electrical line, means for impressing a high voltage low current fault pulse on the line having a magnitude sufficient to are across the fault, means for maintaining said arc-at low voltage, means for periodically impressing a fault locating pulse on the line, means connected to the line for receiving the fault locating pulse after reflection from the line and means, including a cathode ray tube having means for producing a cathode beam and means for controlling the movement of said beam for indicating the time of travel of the last mentioned pulse to the fault arc and reflection back to the point of impression a location of the position of the fault on the line.

9. Ina system for locating a fault on an electrical line, means for impressing a high voltage low current fault pulse on the line sufficient to are across the fault, means for maintaining said are of low voltage; means for periodically iil.- pressing a fault-locating pulse on the line, means connected to the line for receiving the fault locating: pulse after reflection from the line, and forindicatingthetimeof .travel'of the last men.-

tioned pulse to the fault arc and reflection back to the point of impression as a location of the position of the fault on the line, including a cathode ray oscilloscope tube having means for producing a beam, means for repeatedly sweeping said beam across said cathode ray tube, and means operative on said beam to produce indications therewith by which said repeated measurements are indicated and made visible.

10. A system for locating a fault on an electrical line comprising a blocking oscillator for providing spaced electrical pulses of the order of one microsecond at repeated intervals of a substantially lower order of magnitude, gate c'ircuit means operatively controlled by said blocking oscillator for enlarging the time of duration of said pulse, a power pulsing circuit connected thereto and controlled thereby for impressing upon the line connected thereto the fault testing electric pulse, a receiver circuit means having connections to said line for receiving the pulses reflected from the fault on the line at the point of impression, a cathode ray oscilloscope having two pair of deflecting plates, and means for producing and suppressing a cathode ray beam, means connecting the receiver to one pair of de fleeting plates, a sweep circuit means operatively connected to and controlled by said gate circuit means and connected to said other pair of plates for sweeping the beam across the oscilloscope, said gate circuit means connected to the means for producing and suppressing said beam for controlling the time interval of operation between successive pulses of the blocking oscillator.

11. A device as in claim 10, in which the gate circuit comprises means for producing a square top wave with the forward side thereof synchronized with the pulse impressed upon the line and the rearward side thereof marking the termination of operation of the system.

12. A device as in claim including a range marking circuit operatively controlled by said gate circuit and connected to the same pair of plates as the receiver circuit for marking indi- 10 vidual timing marks corresponding to distance on said cathode ray oscilloscope.

13. In a system for locating a fault on an electric line, a visual cathode ray oscilloscope tube having means for producing a beam, means for producing a linear travel of the beam in dia metrically opposite directions, means for producing a travel of the beam at right angles to the linear travel, a gate circuit means, a range marker means operatively included in the means for producing a travel of the beam at right angles to said linear travel for producing a series of marks with the beam on said cathode ray tube, a pulse producing means connected with the said line and sending pulses out on the line, means connected to the line for receiving the pulses after reflection from the line and operatively connected to the oscilloscope for indicating a time measurement, a sharp peaked oscillator operatively connected to the gate circuit means and means connecting the gate circuit means to the range marker means, and operative by the gate circuit wave for commencing the production of marking pulses with the beginning of the gate circuit wave and ending the production with the ending of the gate circuit wave, said means for producing a linear travel of the beam in diametrically opposite directions including a sweep circuit means operatively connected to the gate circuit.

ANDRE J. DEVOT.

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